Jayson @ Chippeduk has shown to do some pretty good work the last time some E30's went down there.
Eprom remaps generally cost around Ԛ£300-Ԛ£400. Those using Unichip cost around Ԛ£500-Ԛ£550.
What gains you'll see - all depends on how good your engine is and how well everything is setup. If everything is fine, it's then a question of how far the guy doing the remap takes the engine.
I would recommend you make sure your car is running 100%. Get a dyno done and see what the fuelling is like.